1. Field of the Invention
This invention relates generally to a method and apparatus for the control of oil and gas production wells. More particularly, this invention relates to a method and apparatus for automatically controlling petroleum production wells downhole using modeling techniques.
2. The Prior Art
The control of oil and gas production wells constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved as well as the risks associated with environmental and safety issues.
Production well control has become particularly important and more complex in view of the industry wide recognition that wells having multiple branches (i.e., multilateral wells) will be increasingly important and commonplace. Such multilateral wells include discrete production zones which produce fluid in either common or discrete production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well.
Before describing the current state-of-the-art relative to such production well control systems and methods, a brief description will be made of the production systems, per se, in need of control. One type of production system utilizes electrical submersible pumps (ESP) for pumping fluids from downhole. In addition, there are two other general types of productions systems for oil and gas wells, namely plunger lift and gas lift. Plunger lift production systems include the use of a small cylindrical plunger which travels through tubing extending from a location adjacent the producing formation down in the borehole to surface equipment located at the open end of the borehole. In general, fluids which collect in the borehole and inhibit the flow of fluids out of the formation and into the wellbore, are collected in the tubing. Periodically, the end of the tubing is opened at the surface and the accumulated reservoir pressure is sufficient to force the plunger up the tubing. The plunger carries with it to the surface a load of accumulated fluids which are ejected out the top of the well thereby allowing gas to flow more freely from the formation into the wellbore and be delivered to a distribution system at the surface. After the flow of gas has again become restricted due to the further accumulation of fluids downhole, a valve in the tubing at the surface of the well is closed so that the plunger then falls back down the tubing and is ready to lift another load of fluids to the surface upon the reopening of the valve.
A gas lift production system includes a valve system for controlling the injection of pressurized gas from a source external to the well, such as another gas well or a compressor, into the borehole. The increased pressure from the injected gas forces accumulated formation fluids up a central tubing extending along the borehole to remove the fluids and restore the free flow of gas and/or oil from the formation into the well. In wells where liquid fall back is a problem during gas lift, plunger lift may be combined with gas lift to improve efficiency.
In both plunger lift and gas lift production systems, there is a requirement for the periodic operation of a motor valve at the surface of the wellhead to control either the flow of fluids from the well or the flow of injection gas into the well to assist in the production of gas and liquids from the well. These motor valves are conventionally controlled by timing mechanisms and are programmed in accordance with principles of reservoir engineering which determine the length of time that a well should be either "shut in" and restricted from the flowing of gas or liquids to the surface and the time the well should be "opened" to freely produce. Generally, the criteria used for operation of the motor valve is strictly one of the elapse of a preselected time period. In most cases, measured well parameters, such as pressure, temperature, etc. are used only to override the timing cycle in special conditions.
It will be appreciated that relatively simple, timed intermittent operation of motor valves and the like is often not adequate to control either outflow from the well or gas injection to the well so as to optimize well production. As a consequence, sophisticated computerized controllers have been positioned at the surface of production wells for control of downhole devices such as the motor valves.
In addition, such computerized controllers have been used to control other downhole devices such as hydro-mechanical safety valves. These typically microprocessor based controllers are also used for zone control within a well and, for example, can be used to actuate sliding sleeves or packers by the transmission of a surface command to downhole microprocessor controllers and/or electromechanical control devices.
The surface controllers are often hardwired to downhole sensors which transmit information to the surface such as pressure, temperature and flow. This data is then processed at the surface by the computerized control system. Electrically submersible pumps use pressure and temperature readings received at the surface from downhole sensors to change the speed of the pump in the borehole. As an alternative to downhole sensors, wire line production logging tools are also used to provide downhole data on pressure, temperature, flow, gamma ray and pulse neutron using a wire line surface unit. This data is then used for control of the production well.
There are numerous prior art patents related to the control of oil and gas production wells. In general, these prior patents relate to (1) surface control systems using a surface microprocessor and (2) downhole control systems which are initiated by surface control signals.
The surface control system patents generally disclose computerized systems for monitoring and controlling a gas/oil production well whereby the control electronics is located at the surface and communicates with sensors and electromechanical devices near the surface. An example of a system of this type is described in U.S. Pat. No. 4,633,954 ('954) to Dixon et al. The system described in the '954 patent includes a fully programmable microprocessor controller which monitors downhole parameters such as pressure and flow and controls the operation of gas injection to the well, outflow of fluids from the well or shutting in of the well to maximize output of the well. This particular system includes battery powered solid state circuitry comprising a keyboard, a programmable memory, a microprocessor, control circuitry and a liquid crystal display. Another example of a control system of this type is described in U.S. Pat. No. 5,132,904 ('904) to Lamp. The '904 patent discloses a system similar to the '954 patent and in addition also describes a feature wherein the controller includes serial and parallel communication ports through which all communications to and from the controller pass. Hand held devices or portable computers capable of serial communication may access the controller. A telephone modem or telemetry link to a central host computer may also be used to permit several controllers to be accessed remotely.
U.S. Pat. No. 4,757,314 ('314) to Aubin et al describes an apparatus for controlling and monitoring a well head submerged in water. This system includes a plurality of sensors, a plurality of electromechanical valves and an electronic control system which communicates with the sensors and valves. The electronic control system is positioned in a water tight enclosure and the water tight enclosure is submerged underwater. The electronics located in the submerged enclosure control and operate the electromechanical valves based on input from the sensors. In particular, the electronics in the enclosure uses the decision making abilities of the microprocessor to monitor the cable integrity from the surface to the well head to automatically open or close the valves should a break in the line occur.
The downhole control system patents generally disclose downhole microprocessor controllers, electromechanical control devices and sensors. Examples include U.S. Pat. Nos. 4,915,168 ('168) to Upchurch and 5,273,112 ('112) to Schultz. However, in each and every case, the microprocessor controllers transmit control signals only upon actuation from a surface or other external control signal. There is no teaching in any of these patents that the downhole microprocessor controllers themselves may automatically initiate the control of the electromechanical devices based on preprogrammed instructions. Similarly, none of the aforementioned patents directed to microprocessor based control systems for controlling the production from oil and gas wells, including the aforementioned '954, '904 and '314 patents, disclose the use of downhole electronic controllers, electromechanical control devices and sensors whereby the electronic control units will automatically control the electromechanical devices based on input from the sensor without the need for a surface or other external control signal.
It will be appreciated that the downhole control system of the types disclosed in the '168 and '112 patents are closely analogous to the surface based control systems such as disclosed in the '954, '904 and '314 patents in that a surface controller is required at each well to initiate and transmit the control instructions to the downhole microprocessor. Thus, in all cases, some type of surface controller and associated support platform at each well is needed.
While it is well recognized that petroleum production wells will have increased production efficiencies and lower operating costs if surface computer based controllers and downhole microprocessor controller (actuated by external or surface signals) of the type discussed hereinabove are used, the presently implemented control systems nevertheless suffer from drawbacks and disadvantages. For example, as mentioned, all of these prior art systems generally require a surface platform at each well for supporting the control electronics and associated equipment. However, in many instances, the well operator would rather forego building and maintaining the costly platform. Thus, a problem is encountered in that use of present surface controllers require the presence of a location for the control system, namely the platform. Still another problem associated with known surface control systems such as the type disclosed in the '168 and '112 patents wherein a downhole microprocessor is actuated by a surface signal is the reliability of surface to downhole signal integrity. It will be appreciated that should the surface signal be in any way compromised on its way downhole, then important control operations (such as preventing water from flowing into the production tubing) will not take place as needed.
In multilateral wells where multiple zones are controlled by a single surface control system, an inherent risk is that if the surface control system fails or otherwise shuts down, then all of the downhole tools and other production equipment in each separate zone will similarly shut down leading to a large loss in production and, of course, a loss in revenue.
Still another significant drawback of present production well control systems involves the extremely high cost associated with implementing changes in well control and related workover operations. Presently, if a problem is detected at the well, the customer is required to send a rig to the well site at an extremely high cost (e.g., 5 million dollars for 30 days of offshore work). The well must then be shut in during the workover causing a large loss in revenues (e.g., 1.5 million dollars for a 30 day period). Associated with these high costs are the relatively high risks of adverse environmental impact due to spills and other accidents as well as potential liability of personnel at the rig site. Of course, these risks can lead to even further costs. Because of the high costs and risks involved, in general, a customer may delay important and necessary workover of a single well until other wells in that area encounter problems. This delay may cause the production of the well to decrease or be shut in until the rig is brought in.
Still other problems associated with present production well control systems involve the need for wireline formation evaluation to sense changes in the formation and fluid composition. Unfortunately, such wireline formation evaluation is extremely expensive and time consuming. In addition, it requires shut-in of the well and does not provide "real time" information. The need for real time information regarding the formation and fluid is especially acute in evaluating undesirable water flow into the production fluids.